This invention relates to certain polyethylene-polystyrene blend compositions and to the preparation of closed cell foam products therefrom. In a more particular aspect, this invention relates to a novel foam product having heretofore unattainable structural indices and properties.
Heretofore, polystyrene resins and polyethylene resins have individually been used in the form of molded foams, which have applications such as heat insulation materials, shock absorbing materials, floating materials, etc.
However, such applications have thus far been limited to those of specific conditions conforming to the properties peculiar to the individual foam products concerned. More specifically, polystyrene resin foams are deficient in solvent resistance, flexibility, compressive recovery, and capacity in absorbing repeated impacts and are thus not generally suitable as shock absorbing cushioning materials. On the other hand, polyethylene resin foams are deficient in stiffness, thermal resistance, and creep resistance and are therefore not generally suitable for applications requiring such properties.
Efforts have heretofore been made to blend the two resins and develop a unique foam combining the advantageous properties of the two resins. For example, the most advanced ideas are found in Japanese Patent Publication No. 34,662/1977 dislcosing a method of mixing the two resins in the presence of a specific mixed medium and Japanese Patent Laying-Open No. 35,471/1974 describing a method of polymerizing styrene as contained in polyethylene resin; both of these methods being ultimately directed toward foaming the combined resins.
These and other conventional techniques, however, are not believed to have reached suitable levels of completion.
This is because of the fact that, even though the foams obtained according to these conventional methods, as compared with the foams of polystyrene, are improved in enhancement of flexibility, they are nonetheless not at all improved in repeated elastic recovery or in repeated cushioning coefficient (average maximum acceleration: G value in JIS Z No. 0235, 6.5.1.). Furthermore, the foams resulting from the aforementioned conventional blend approaches, as compared with the foams of polyethylene resin, exhibit undesirable properties such as increased stiffness and degraded solvent resistance or lowered repeated cushioning coefficient (G value). These conventional polystyrene-polyethylene foams are low in closed cell ratio, so their heat insulation properties are inferior. When the foams are subjected to fabrication by cutting and severing, there frequently ensues heavy occurrence of burrs on cut and severed surfaces, i.e., a phenomenon which symbolizes the difficulty in the homogenization of the two resins. This phenomenon tends to gain in prominence in proportion as the expansion ratio of the blends is increased.
In recent years, the demand has been increasing in the market for cushioning materials suiting the packaging of light-weight commodities such as audio instruments, compact computers, and high-grade cameras, all of which abhor repeated shocks and vibrations while in transportation as well as for flexible materials having both noise insulation property and heat insulation property.
Properties desirable for cushioning materials include flexibility enough to absorb small vibrations, stiffness enough to resist buckling or creeping on exposure to relatively large shocks and loads, combination of flexibility and stiffness in economical thickness, and stability to retain these properties at such a temperature of about 70.degree. C. encountered in the holds of ships, for example. Properties desirable for heat insulation materials include flexiblity enough to be bent and compressed so as to be packed in walls, heat insulation property high enough to enhance the inhabitability in economical thickness, flexibility and heat insulation retaining property, flexibility enough to absorb vibrations generated on the floors, in cases where the heat insulation materials are used in floating floors, stiffness enough to withstand the loads applied, dimensional stability in temperatures exceeding about 70.degree. C., low water vapor permeability and outstanding heat insulation.
These requirements (i.e., the provision of the above-stated combination of properties) are not satisfied by conventional polyethylene or polystyrene foamed products nor by the foam products provided by the conventional polyethylene-polystyrene blend techniques. Consequently, it has been costly to manufacture individual materials meeting the specific requirements of various end-use applications.
The primary object of this invention, therefore, is to provide a novel foam which exhibits a well balanced combination of flexibility and compression strength, excellence in compression recovery, resistance to distortion due to repeating compression, resistance to creep, heat insulation property, durability or good retention of heat insulation property as a function of time, and suitability for fabrication by cutting and severing, and dimensional stability against heat and solvents.